how i can texture paint this flat face of my model?
I have had similar problems, It could be that the UV is not &quot;flat&quot;, but it does not look like it from the picture. the other experience I had a distortion because the model was too low poly, from the UV editor picture It looks like the face is an N-gon, you could try adding more geometry to it.
As for exporting the UV lines to paint in another software you can Export the UV layout, there is an answer here.